1. Field of the Invention
The natural heat available in the planet Earth represents an enormous energy source which is known as geothermal energy. This heat exists subterraneously at high temperatures as the result of radioactivity decay processes taking place in the earth's mantle and deeper in the molton core of the earth. Volcanic eruptions bring relatively small amounts of that heat source to the surface. Hot springs which result from surface water intrusions into faults and natural fractures in the subterranean hot rock are indicative of a hydrothermal reservoir. Of all the accessable natural energy sources found on earth, by far the largest is geothermal energy and in particular that form which is stored in subterranean hot rock strata of the earth's crust. The term "rock" herein refers to any naturally occurring solid matter, be it igneous, mineral, salt or otherwise, wet or dry. The available energy stored in subterranean hot rock is estimated to be 300 times larger than that obtainable from combustion of all known reserves of fossil energy fuels, i.e., oil, gas and coal.
2. Description of Related Art
Geothermal heat recovery systems employed to date require thermal energy to be removed from hot water and/or steam that is naturally occurring in a subterranean hydrothermal reservoir or aquifer. Such hot water and steam are brought to the surface where the steam is used to drive a turbine and the hot water is flashed to produce steam which in turn is used to drive a turbine and condensed. In instances where the hydrothermal reservoir is not capable of providing sufficient steam for use in a turbine or where the aquifer contains high concentrations of dissolved solids and/or of noncondensable gases, a binary cycle may be advocated whereby the hot fluids from the hydrothermal reservoir are passed through a heat exchanger to transfer its thermal energy to vaporize a secondary liquid, e.g., isobutane or other suitable organic liquid having a boiling point lower than that of water, and the vapor of this secondary liquid is then passed through a turbine, condensed to liquid and recycled in a closed circuit. Hydrothermal waters have also been employed in heat exchanger systems in a variety of space heating and drying operations. Usually, the heat depleted hydrothermal fluids are reinjected into the underground reservoir. In certain instances they are in whole or in part dispersed to the environment. Generally the hydrothermal fluids contain dissolved minerals which cause numerous mechanical and operational problems in the heat recovery equipment. Often, the hydrothermal fluids contain hazardous and noxious substances, and discharging the heat depleted fluids can be detrimental to public heath and safety. In places where mobile undergound hot water occurs at relatively shallow depths, submerged downhole heat exchangers have been used to recover relatively low temperature heat for space heating and the like.
To date, efforts to recover thermal energy from subterranean hot rock have been limited to experimental systems based on fracturing the underground rock to create permeability and injecting water into the fracture zone to form an artificial hydrothermal reservoir that acquires heat by conduction from the surrounding hot rock. In such a system, a flow of cold water from the surface is injected into one side of the fracture zone and a flow of heated water is withdrawn from the opposite side of the fracture zone and brought to the surface. The thermal energy available in the heated water returned from the artificial hydrothermal reservoir is recovered by procedures similar to those employed with the natural hydrothermal reservoirs aforesaid and the so cooled water is reinjected into the fracture zone. In such systems, the water in the artificial reservoir can dissolve or otherwise transport mineral s that are present in the rocks and cause mechanical and operational problems similar to those experienced with natural hydrothermal heat recovery systems.
A conceptual design for recovering heat from hot rock had been reported which would remove heat from hot rock at a depth of 50,000 feet (approximately 10 miles) by the circulation of water in a closed liquid phase system consisting of a single pipe well in which is suspended an insulated coaxial smaller diameter pipe. A circulation of cold water from the surface passes down the outer annulus, acquires heat transferred by conduction from the surrounding hot rock and is returned from the bottom to the surface through the inner coaxial pipe. The thermal energy available in the returned hot water is recovered by heat exchange to an external circulation. (J. H. Warren and R. L. Whitelaw, Design of an Insulated Coaxial Pipe Assembly for a Drilled Geothermal Well, AIChE-ASME Heat Transfer Conference, San Francisco, Calif., Aug. 11-13, 1975). The report acknowledges that this concept presents a number of engineering and design problems such as heat recovery inefficiency because of the close proximity of hot and cold fluids in coaxial counterflow, mechanical difficulties because of the differential in thermal expansion between the liners of the coaxial pipe assembly, construction and operational problems due to the considerable hydrostatic pressures developed at the 50,000 feet depth of the well, and the questionable feasibility of insulating, supporting and installing the long coaxial pipe assembly.
The increase of temperature with depth in the earth's crust is known as the geothermal gradient. 30 degrees Celsius per kilometer of depth (".degree.C./km.") is considered to be the average for all surface locations and 90.degree. C./km is considered to be a moderately high geothermal gradient known to exist at many surface locations where subterranean depths of about 20,000 feet can be reached with present day drilling techniques. Areas of vulcanism or magmatic intrusion can provide anomalies with higher geothermal gradients.